YICHANG -- Police are questioning the owner of a power plant where a huge blast killed 21 people and left five injured in central China's Hubei Province, authorities said Friday.

Twenty-one people, aged between 22 and 51 years old, died in Thursday's explosion at Madian Gangue Power Generation Co Ltd. in Dangyang City. DNA testing will be used to identify the bodies, local authorities said.

The power plant and neighboring companies have been closed and a work safety overhaul has been launched in the city.

Four of the injured were transferred to the provincial capital Wuhan by helicopter for treatment.

The blast occurred at about 3:20 p.m. Thursday when workers were testing a newly-built thermo-power facility. The blast was the result of a high-pressure steam pipe bursting, preliminary investigations showed.

Ma Xuming, mayor of Yichang City, said several work groups were established to handle rescue, investigation and to comfort relatives of the victims.

The State Administration of Work Safety has sent a work group to oversee the rescue operation and conduct an investigation.